USGS Sea Otter Survey in California
- Brian Hatfield
United States Geological Survey (USGS)
- Jim Estes
United States Geological Survey (USGS)
- Tim Tinker
University of California, Santa Cruz
The following information is from the USGS Western Ecological Research Center. Go to the following URL to get further details:
Bi-annual aerial and land-based standardized surveys of Southern sea otters have been conducted in California (from Santa Barbara to Half Moon Bay) during late spring and early fall, since 1983. The surveys are a collaborative effort involving the U.S. Geological Survey, the California Department of Fish and Game, the U.S. Fish and Wildlife Service, and the Monterey Bay Aquarium with the help of experienced volunteers.
The surveys record the total otter numbers, the number of dependent pups, and the number of adults and sub-adults, or independents observed. Spring survey results are used as an indicator of the population trend of California sea otters. The scientists also use running (3-year) averages of the results to assess whether the population is growing or declining. Each value or point plotted on a graph is the average of count numbers from three consecutive years (e.g., the value for 2001 is the average of the 2000, 2001, and 2002 counts.).
Annual spring surveys of sea otters indicate a growth rate of about 5 percent until 1995. Since 1995, the rate of growth has slowed to an average of less than 1 percent per year. These data, in conjunction with findings from other studies provide the necessary information to assess female reproductive rates and changes in reproductive success of the California sea otter population through time. The information gathered will also be used by federal and state wildlife agencies in making decisions about the management of the Southern sea otter.
Summary to DateThe spring 2013 sea otter count began on 3 May. All of the ground-counted areas and half of the aerial sections were finished by 16 May, but poor weather conditions and plane availability precluded the completion of the southern aerial portions until 4 June. Overall viewing conditions this year were slightly less favorable than those during the 2012 spring census (2.5 vs. 2.7, where 0=poor, 1=fair, 2=good, 3=very good, and 4=excellent). The surface kelp canopies were considered to be about normal for this time of year in most areas of the otter’s central range. The region surveyed spans the mainland coastline from Point San Pedro, San Mateo County in the north to Rincon Point at the Santa Barbara/Ventura County line. The small population at San Nicolas Island is monitored by a separate survey effort, details of which are provided below.
The total count of the mainland population was almost unchanged from the 2012 census. The number of pups counted this spring represents a record high, while the number of independent sea otters (juveniles and older) was actually down slightly, with the result that the number of pups per 100 adults (18.6) was the highest since 1990. It should be emphasized that there is a considerable degree of uncertainty (random variation due to sampling and measurement error) in any census count, and thus longer-term trends are far more informative than year-to-year differences. The U.S. Fish and Wildlife Service’s Southern Sea Otter Recovery Plan specifies that the 3-year running average of the total count be used as the official metric for monitoring trends of the mainland population, thereby reducing the influence of anomalously high or low counts from any particular year. The trend in the mainland population over the past 5-7 years remains essentially flat, with a mean annual growth rate of just 0.16%; however, regional trends vary considerably. In the north part of the range (Pigeon Pt. – Seaside) the number of independent otters counted was down this year but the longer-term trend remains positive. In the center portion of the range (Seaside to Cayucos), where sea otter densities are highest, the number of independent sea otters counted was up this year compared to last, and there is a slightly positive trend here too. In contrast, the number of independent sea otters counted south of Cayucos was down for the third year in a row and there is a relatively strong negative trend, particularly between Pismo Beach and Pt. Sal. This latter trend is consistent with the record number of carcasses recovered from the southern portion of the range, over half of which were associated with shark-bite mortality, and suggests that the increased rate of shark attacks in recent years is in large part responsible for declining numbers in this area.
Only one sea otter was observed north of Pigeon Point and that was an animal seen approximately 3 km north of the Point. At the south end of the range, 68 otters were counted southeast of Point Conception (11 fewer than last spring), with only one animal spotted southeast of Gaviota State Beach (near Naples Reef). The limits of the sea otter’s range along the mainland coast remain approximately unchanged from 2012: the northern boundary is considered to be approximately 2 km southeast of Pigeon Pt., while the southern boundary is considered to be near the Agua Caliente Creek mouth approximately 3 km west of Gaviota State Beach. The mainland range limits are defined as the points farthest from the range center (to the north and south) at which 5 or more otters are counted within a 10 km contiguous stretch of coastline (as measured along the 10 m bathymetric contour) during the two most recent spring censuses, or at which these same criteria were met in the previous year.
- California sea otter numbers are up, according to the latest population survey led by federal, state and university scientists. The reasons: more pups — and the addition of San Nicolas Island sea otters to the population count.
- For the 2013 report, USGS lists the population index as 2,941. For southern sea otters to be considered for removal from threatened species listing, the population index would have to exceed 3,090 for three consecutive years, according to the threshold established under the Southern Sea Otter Recovery Plan by the U.S. Fish and Wildlife Service.
- “Population growth in central California has faltered recently, so the fact that we’re seeing a slightly positive trend is a basis for cautious optimism,” says Tim Tinker, a biologist with the USGS Western Ecological Research Center who supervises the annual survey. “Certainly, sea otters have made an impressive recovery in California since their rediscovery here in the 1930s. But as their numbers expand along California’s coast, they are facing different ‘growing pains’ in different locales. Our research partnership is investigating the factors responsible for these local trends.” “We counted a record number of pups this year, which led to the uptick in the 3-year average,” says USGS biologist Brian Hatfield, coordinator of the annual survey, “A high pup count is always encouraging, although the number of adult otters counted along the mainland was almost identical to last year’s count, so we’ll have to wait and see if the positive trend continues.”
- There is a second reason for the higher population index reported this year. In 2013, the equation for this population index was amended to add sea otters living at San Nicolas Island. One-hundred-and-forty sea otters were introduced to the island in the 1980s as part of a USFWS recovery experiment, but most of them returned to the mainland, died, or simply disappeared. USFWS completed an extensive review of the translocation program in December 2012, resulting in termination of the program. As a consequence, sea otters at San Nicolas Island are no longer considered to be an “experimental” population and will now be included as part of the California-wide population index for southern sea otter recovery. The population at the island is now at 59 individuals.
- Age structure
Study MethodsBi-annual land- and aerial-based survey with 3 year averages.
Figures and Images
Figure 3. Sea otter population trend from 1984 to present, and both delisting and uplisting thresholds. Number of sea otters based on a three-year running average.